As notebooks, cell phones, hand held devices and the like continue to grow in popularity, so does the need for power supplies to these devices. To gratify this need, these devices are provided either with single input plug-in power adapters or dual input chargers that manage charging of batteries from two power sources, typically via a wall adapter and a USB interface. A charger typically charges a battery and at the same time generates regulated power for use within a system. Numerous battery charger architectures have been developed in recent years. Normally, a charger system is a step-down converter that drives a lower output load like a battery using a higher input supply.
A dual input charger known in the art comprises load switch integrated circuits (ICs) implementing transistors, typically, MOSFETs. MOSFETs usually include an inherent parasitic diode i.e. a body diode, which is formed between the drain and the source of the MOSFET. To receive power from the two input sources, dual input chargers use two load switch ICs, one each at the receiving end of each of the input power sources. Each of the load switch ICs comprises a series combination of two nMOSFETs in a source-drain-drain-source configuration. The load switch ICs are provided with an ‘enable’ signal to turn ON a desired load switch IC. The load switch ICs, as described herein above, when used in combination with a switching/linear charger find applications in dual input single output chargers. A switching/linear charger further comprises a series combination of two nMOSFETs in a source-drain-drain-source configuration.
Therefore conventional dual input chargers providing a single output require at least six or seven nMOSFETs for proper functioning, thereby increasing the cost and making the system complex because of the need for additional circuitry for driving the large number of connected switches in an optimum manner.
One of the products of Texas Instruments, BQ24160 (dual input switching charger) implements the aforementioned architecture to receive input from two power sources and provides a single output. The system comprises a pair of load switches, one connected to each input power source. Each load switch further comprises a pair of nMOSFET switches connected to each other in a source-drain-drain-source configuration. The load switches are connected to a linear/switching charger that also implements a series combination of two nMOSFETs in the source-drain-drain-source configuration. In a single input linear charger application, the configuration of a product of Texas Instruments, BQ25040 involves use of six nMOSFET switches (4 switches in operation during a charging mode) and in a single input switching charger application, the configuration of a product of Texas Instruments, BQ24150 would involve use of seven nMOSFET switches (5 switches in operation during a charging mode) for implementing a dual input single output functionality. The increased number of switches and additional control signals for selecting a specific load switch increases the complexity of the circuit. Moreover, an increased number of series connections of nMOSFETs increases conduction losses, thereby reducing efficiency and increasing cost.
Another endeavor towards providing a dual input single output charger is disclosed in U.S. Pat. No. 7,759,907B2 wherein the system receives power from two input power sources and is provided with a source selector circuit to logically select one of the two power sources. The source selector circuit further comprises two circuits implementing a combination of isolation diodes and bypass transistors, each of the circuits being associated with one input power source. These bypass transistors are p-type transistors, that together with pull up resistors and isolation diodes perform selection of an input power source and connect the selected input power source to the output. The aforementioned source selector circuit by itself makes use of four bypass transistors, isolation diodes and pull up resistors. When used in a charging application, the circuit involves further components that add to conduction losses and accordingly the cost of the system increases.
Hence, there is felt a need for an improvised charging circuit arrangement which has improved efficiency, is less complex and results in a cost effective system.